OUMUAMUA: Is the first interstellar asteroid finally a comet ?

Discovered on October 17, 2017 , Oumuamua (or 1U/2017 U1) was at the top of the scientific news since it became the first known foreign celestial object coming from another star observed in our solar system. Initially designated as a comet (C/2017 U1), scientists had modified its appellation on October 26th 2017. Since then, it was considered as an asteroid because any coma was detected. However, a recent study has just been published claiming that Oumuamua could be a comet after all!

Particles and jets outgassing

On June 27th 2018, an official paper came out in the world’s famous scientific journal “Nature”, explaining that the asteroid would release tiny amounts of dust and gas, enough to propel and modifying its motion, speed and rotation. Such a phenomenon occurs mostly in comets.

This striking discovery gathering international scientists is the outcome of astronomical observations led with the most powerful telescopes of the world: the Hubble space telescope, the ground-based observatories like Canada-France-Hawaii, Gemini South and the Very Large Telescope (VLT) in Chile.

To get to this conclusion, astronomers tried at first to characterize the asteroid’s trajectory using the celestial mechanics which is affected by the gravitational forces of the Sun and the planets. Stunningly, they noticed that the theoretical position of Oumuamua has an error of 40 000 kilometers and its speed is less than what observations tell.

Through studying in details the evolution of its position, this international team of astronomers whose lead author is Marco Micheli (European Space Agency) could find that something inside the asteroid itself should have sped it up and derived it. Scientists did not expect to this internal force since no trail (coma) was actually observed in the pictures. Even though they are relatively weak, these gas and dust jetswould have been released in enough amounts to serve as natural propellant on the asteroid. As a whole, such activity is only observed on comets and with a far more important intensity, which makes it visible through telescopes.

As Oumuamua is currently the first interstellar object of its kind to enter our solar system, to declare whether it is a comet or an asteroid is still a delicate question. The official scientific paper does not end with an unanimous decision, even if it would have comet-like features. Astronomers hope to discover other celestial bodies similar to Oumuamua in order to compare the results and to deduce the exact true nature of this mysterious asteroid.

An Asteroid which has always been keeping astronomers curious

Oumuamua is a fascinating object, which does not unveil all of its secrets. At the moment of its discovery, astronomers had already measured its extremely elongated trajectory, for which any other ones had been observed in the solar system. This characteristic called eccentricity is equal to 1.19, meaning that the motion of Oumuamua is heavily elliptic whereas most of comets or asteroids have an excentricity around 0.2-0.7 whose orbit can be considered as circular. Because of this peculiar orbit, scientists had declared that Oumuamua was likely to come from another planetary system towards the Lyra constellation.